When we talk about neurons in the brain, we rarely realize how different they are. That is, we know that they use various neurotransmitters, someone will remember that there are excitatory and inhibitory neurons (which suppress the activity of other neurons). Nerve cells of the cerebral cortex, for example, are also divided into those that extend outside the cortex, and those that form connections only within the cortex. However, in reality, the variety of neurons is much wider, just to cover it in its entirety requires colossal efforts of various specialists.
It is for this that BICCN, the BRAIN Initiative – Cell Census Network, was organized in the system of the National Institutes of Health (NIH) of the United States, a project that literally deals with the census of brain cells (moreover, the word BRAIN here is also an abbreviation – Brain Research through Advancing Innovative Neurotechnologies, that is brain research using promising innovative neurotechnologies). The efforts of more than 400 researchers have borne fruit: in the journal Nature published 17 articles, which describe more than a hundred types of neurons. Moreover, this hundred types have been found so far only in the motor region of the cerebral cortex, that is, in the area that coordinates movements. Researchers have not yet looked into other parts of the brain.
Neurons were rewritten in three brains at once: in the human brain, in the brain of the marmoset monkeys and in the brain of the mouse – this way it was possible to understand the evolutionary changes that took place in the brain of animals at the level of individual cells. The most obvious criteria by which neurons can be classified are their location, shape, and function. Location is understood not only as “registration” in one or another area of the brain. As you know, the cortex consists of several neural layers-floors, and there are other features of the location of neurons, so their exact coordinate is a rather complex parameter. The function can partly be understood if we trace where the processes of neurons go, from whom they receive signals and where they are sent.
But function is also the nature of electrical activity. A neuron may be prone mainly to high-amplitude or low-amplitude pulses, or high-frequency pulses, etc. The lion’s share of the research effort at BICCN has gone into determining the electrophysiological characteristics of individual cells.
Another important complex of features is the molecular portrait. This is primarily the activity of certain genes. All neurons in the same brain have the same genes, but they work differently. The activity of a gene can be determined by the amount of RNA that is synthesized on it (as we remember, genetic information is first copied from DNA into RNA, and a protein is already synthesized on RNA, or RNA itself acts). The researchers analyzed the activity of 258 genes, while simultaneously determining the location in the motor cortex where a particular neuron with a particular genetic activity is located.
Another important molecular feature of any cell is the state of its epigenetics. Gene activity depends on several molecular processes that are collectively referred to as epigenetic regulation. For example, chemical methyl groups can appear and disappear on DNA, which determines whether a gene will be active, or DNA can be tightly packed with special proteins – so tightly that enzymes that read genetic information cannot work with it. Epigenetic mechanisms work for a long time; they can remain in the same position throughout the life of a cell. Epigenetic traits also need to be taken into account when we begin to systematize neurons.
As we said above, more than a hundred types of neurons were counted, but the exact number here depends on which criterion is chosen as the main one. In addition, a certain hierarchy was found among different cell types. Several types are very clearly delineated – their cells show the same properties regardless of how we look at them, what method we study. And there are other types of cells, the boundaries between which are blurred. That is, we, for example, have clearly two different types of neurons, if we look at many nerve cells in a large population, but at the same time we come across many such neurons that, on the one hand, belong to one type, and on the other hand, to another type.
But, one way or another, the significance of the work is still such that the portal Science calls the new atlas of neurons the “Rosetta Stone of Neurobiology” (recall that the Rosetta Stone helped to decipher the meanings of ancient Egyptian hieroglyphs). With such a detailed classification of neurons, it is possible, for example, to answer an extremely interesting question, how does a nerve cell change with age – say, can a neuron, having studied something for a long time, switch from one type to another? And, of course, with new types of neurons and new methods that have been developed to do this, it will be easier to study neurons in other areas of the brain.
Source: Автономная некоммерческая организация "Редакция журнала «Наука и жизнь»" by www.nkj.ru.
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